The present invention relates to the field of scanned beam systems, and in particular, to a method and apparatus for tilting a beam column.
Scanned beam microscopy systems, including charged particle beam systems such as electron beam and focused ion beam (FIB) systems, are widely used in characterization or treatment of materials on a microscopic scale. For example, focused ion beam systems are used in manufacturing operations because of their ability to image, etch, mill, deposit and analyze with great precision. Ion columns in FIB systems using gallium liquid metal ion sources (LMIS), for example, can provide five to seven nanometer lateral imaging resolution.
The beam of a scanning beam system typically scans the surface of a target specimen in a raster pattern. This raster pattern may be used to produce an image of the surface of the target. When the scanned beam strikes the target, particles or photons are emitted from the immediate vicinity of beam impact. A portion of these emissions are measured or collected using a suitable detector or collector that produces an output signal indicative of the intensity of the emission. This output signal is then processed to produce an observable image displayed on a conventional video monitor.
A typical application of scanning beam systems is for analysis and treatment of integrated circuits (IC). In this application, a focused ion beam is used to produce an image of the circuit. This image is then used in conjunction with circuit layout information to navigate the ion beam over the surface of the circuit to locate a specific element or feature of interest. When the beam is scanned to the local area of interest, the beam current can be increased to cut into the circuit die and expose circuit features buried in layers. The FIB system can then alter the exposed circuit by cutting conductive traces to break electrical connections or by depositing conductive material to provide new electrical connections. This etching or deposition is caused by a physical or chemical reaction of the beam ions with the specimen and occurs at a rate that is largely dependent upon the constituent ions of the beam, the presence and type of etch enhancing or deposition precursor gases, and the beam current.
Although the typical focused beam system configuration provides a beam that impinges normal to the substrate, focused beam systems may be used in tilt orientations, in which the beam impinges at an off-normal angle of incidence with respect to the plane of the substrate to perform ion beam milling or electron beam viewing at a specified angle. Although this could be accomplished by tilting the stage that contains the working piece to be viewed or etched, there is difficulty in maintaining coincidence between the center point of beam impact and the axis of stage rotation for all desired angles of incidence.
Alternatively, a change in angle of incidence could be obtained by tilting the beam column about an axis of rotation passing through the working piece at the desired center point of beam impact. But prior art methods do not provide a satisfactory way to provide a change in column tilt angle without interrupting system operation. Prior art methods for providing column tilt to produce an off-normal angle of incidence include the use of fixed tapered spacers in conjunction with vacuum seals to set the incidence angle of the beam. To expose the working piece to successive incidence angles, one must iteratively change the spacers used to set the angle of column tilt. Changing the tapered spacers required exposing the sample chamber, thereby requiring additional time to evacuate the chamber and restart and stabilize the emitter after the tilt angle is changed. An alternative prior art method employs a bellows that purportedly enables the system to remain sealed while the column is mechanically tilted, but the change in tilt angle must be performed manually and is difficult to rapidly set to a precise angle of tilt.
It is desirable to have the beam remain focused at the same point on the work piece throughout a range of column tilt angles. This can be achieved by tilting the beam about the point at which the beam is focused to maintain a constant xe2x80x9ceucentric point.xe2x80x9d A xe2x80x9ceucentric pointxe2x80x9d is defined as an arbitrary point through which the beam passes when it is not being deflected and that is a specified distance from the axis of beam deflection. The eucentric point is preferably chosen to coincide with the center point of beam impact and the eucentric point preferably stays at the same location in space for all angles of column tilt. Prior art methods do not provide a satisfactory method of achieving this constant eucentric point. Further, prior art methods do not provide a satisfactory way to provide a change in tilt angle without interrupting system operation. It would therefore be desirable to provide a system and method that enables column tilt over a range of tilt angles during system operation while maintaining a constant eucentric point that overcomes prior art limitations.
The present invention overcomes prior art limitations by providing a method and apparatus for automated adjustment of the tilt angle of a beam column during operation of a scanned beam system over a continuous angular sector, while maintaining a constant eucentric point, maintaining vacuum integrity of the column and work chamber, and without introducing significant vibrations.
According to the present invention, a beam column can be driven electro-mechanically throughout a range of angular displacement to enable precise control of the angle of tilt while maintaining a constant eucentric point. The electro-mechanical drive system can be controlled by computer to provide a desired sequence of angular displacements through which the column is tilted during operation of the beam system. Air bearing support is provided to minimize friction and vibration in the system and a unique bellows is employed to maintain a vacuum or low-pressure environment as the column is tilted
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the disclosure provided herein may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Persons of skill in the art will realize that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims, and that not all objects attainable by the present invention need be attained in each and every embodiment that falls within the scope of the appended claims.